1. Field of the Invention
The present invention provides a circuit for increasing the steepness of the rise of the pulses of a video signal.
2. Brief Description of the Background of the Invention Including Prior Art
Video signals for a defect free reproduction of pictures in a color television set in general have a video band width of about 5 MHz. It is possible in practice that a video signal has a smaller band width of 2 to 3 MHz, for example in the case of the scanning of poor films, in the case of simple cameras, in the case of the reproduction of recording apparatus with a lower band width or in the case with limited transmission sections. Such a lower band width means that during reproduction there is a decreased sharpness of the picture, in particular relating to the vertical edges of the picture.
It is known to improve the sharpness in such a signal by processing the video signal. A correction signal is added to the video signal in differential equalizers, which are employed for this purpose, and the correction signal is obtained by two-fold differentiation of the video signal. This causes a steeper rise of the video signal pulses which run too flat because of the band width limitations.
If in the case of such a circuit the amplitude of the correction signal is of an optimum value for small signal jumps, then in the case of larger signal jumps very high voltage peaks can occur in the corrected video signal, which voltage peaks effect a subjective deterioration of the picture or can interfere with the synchronization.
It is known for the purpose of avoiding these disadvantages from German Patent DE-PS No. 3,141,761 to coordinate to each of the two differentiation stages for obtaining the correction signal an amplitude limiter dimensioned such that in the case of large signals jumps in the video signal there is no pulse peak in the corrected video signal which would lower the quality of the picture reproduction or would interfere with synchronization.
The effect of such a circuit depends on the frequency dependence of the offered video signal. Depending on frequency dependence there can occur an undercompensation or an overcompensation of the video signal. In the case of an undercompensation a sufficient increase in the sharpness of the picture is not achieved, whereas in the case of an overcompensation the over emphasis of the edges can also lead to a subjectively observed deterioration of the picture.
A circuit for increasing the steepness of the rise of the slopes of a video signal is described in the German Patent Application Laid Open DE-OS No. 3,226,432. According to this teaching, a high pass filter is coordinated into the signal path of the correction signal in order to avoid the taking into account of low frequencies in the correction. This occurs in order to avoid the appearance of plastic effects in a picture. The high amplitudes at high frequencies are increased in this case before applying the first differentiation.
A method for the artificial increase of contrast at the sound value jumps and the contours in stereotype plates produced with electronic electrotype machines is taught in the German Patent Application Laid-Out DE-AS No. 1,039,842. In this case, a video signal is limited with an amplitude limiter. This is done in order to obtain sharp edges during printing. The reproduction in this case refers to pictures which do not move. In this case the time constants are very large, and the method and teaching provided in this reference are not suitable for improving the signal jumps of moving pictures.